The present invention relates generally to methods and apparatus for transmitting digital data in network system utilizing cable modems. More specifically, the present invention relates to methods and apparatus for quickly and efficiently locating upstream data channels substantially free of ingress noise or other interference in network systems utilizing cable modems.
The cable TV industry has been upgrading its signal distribution and transmission infrastructure since the late 1980s. In many cable television markets, the infrastructure and topology of cable TV systems now include fiber optics as part of its signal transmission component. This has accelerated the pace at which the cable industry has taken advantage of the inherent two-way communication capability of cable systems. The cable industry is now poised to develop reliable and efficient two-way transmission of digital data over its cable lines at speeds orders of magnitude faster than those available through telephone lines, thereby allowing its subscribers to access digital data for uses ranging from Internet access to cablecommuting.
Originally, cable TV lines were exclusively coaxial cable. The system included a cable headend, ie. a distribution hub, which received analog signals for broadcast from various sources such as satellites, broadcast transmissions, or local TV studios. Coaxial cable from the headend was connected to multiple distribution nodes, each of which could supply many houses or subscribers. From the distribution nodes, trunk lines (linear sections of coaxial cable) extended toward remote sites on the cable network. A typical trunk line is about 10 kilometers long. Branching off of these trunk lines were distribution or feeder cables (40% of the system""s cable footage) to specific neighborhoods, and drop cables (45% of the system""s cable footage) to homes receiving cable television. Amplifiers are provided to maintain signal strength at various locations along the trunk line. For example, broadband amplifiers are required about every 2000 feet depending on the bandwidth of the system. The maximum number of amplifiers that can be placed in a run or cascade is limited by the build-up of noise and distortion. This configuration, known as tree and branch, is still present in older segments of the cable TV market.
With cable television, a TV analog signal received at the headend of a particular cable system is broadcast to all subscribers on that cable system. The subscriber simply needed a television with an appropriate cable receptor to receive the cable television signal. The cable TV signal was broadcast at a radio frequency range of about 50 to 800 MHz. Broadcast signals were sent downstream; that is, from the headend of the cable system across the distribution nodes, over the trunk line, to feeder lines that led to the subscribers. However, the cable system did not have installed the equipment necessary for sending signals from subscribers to the headend, known as return or upstream signal transmission. Not surprisingly, nor were there provisions for digital signal transmission either downstream or upstream.
In the 1980s, cable companies began installing optical fibers between the headend of the cable system and distribution nodes (discussed in greater detail with respect to FIG. 1 below). The optical fibers reduced noise, improved speed and bandwidth, and reduced the need for amplification of signals along the cable lines. In many locations, cable companies installed optical fibers for both downstream and upstream signals. The resulting systems are known as hybrid fiber-coaxial (HFC) systems. Upstream signal transmission was made possible through the use of duplex or two-way filters. These filters allow signals of certain frequencies to go in one direction and of other frequencies to go in the opposite direction. This new upstream data transmission capability allowed cable companies to use set-top cable boxes and allowed subscribers pay-per-view functionality, ie. a service allowing subscribers to send a signal to the cable system indicating that they want to see a certain program.
In addition, cable companies began installing fiber optic lines into the trunk lines of the cable system in the late 1980s. A typical fiber optic trunk line can be up to 80 kilometers long, whereas a typical coaxial trunk line is about 10 kilometers long as mentioned above. Prior to the 1990s, cable television systems were not intended to be general-purpose communications mechanisms. Their primary purpose was transmitting a variety of television signals to subscribers. Thus, they needed to be one-way transmission paths from a central location, known as the headend, to each subscriber""s home, delivering essentially the same signals to each subscriber. HFC systems run fiber deep into the cable TV network offering subscribers more neighborhood specific programming by segmenting an existing system into individual serving areas between 100 to 2,000 subscribers. Although networks exclusively using fiber optics would be optimal, presently cable networks equipped with HFC configurations are capable of delivering a variety of high bandwidth, interactive services to homes for significantly lower costs than networks using only fiber optic cables.
FIG. 1 is a block diagram of a two-way HFC cable system utilizing a cable modem for data transmission. It shows a headend 102 (essentially a distribution hub) which can typically service about 40,000 subscribers. Headend 102 contains a cable modem termination system (CMTS) 104 that is needed when transmitting and receiving data using cable modems. CMTS 104 is discussed in greater detail with respect to FIG. 2. Headend 102 is connected through pairs of fiber optic lines 106 (one line for each direction) to a series of fiber nodes 108. Each headend can support normally up to 80 fiber nodes.
Pre-HFC cable systems used coaxial cables and conventional distribution nodes. Since a single coaxial cable was capable of transmitting data in both directions, one coaxial cable ran between the headend and each distribution node. In addition, because cable modems were not used, the headend of pre-HFC cable systems did not contain a CMTS. Each of the fiber nodes 108 is connected by a coaxial cable 110 to duplex filters 112 which permit certain frequencies to go in one direction and other frequencies to go in the opposite direction (frequency ranges for upstream and downstream paths are discussed below). Each fiber node 108 can normally service up to 500 subscribers, depending on the bandwidth. Fiber node 108, coaxial cable 110, two-way amplifiers 112, plus distribution amplifiers 114 along trunk line 116, and subscriber taps, i.e. branch lines 118, make up the coaxial distribution system of an HFC system. Subscriber tap 118 is connected to a cable modem 120. Cable modem 120 is, in turn, connected to a subscriber computer 122.
Recently, it has been contemplated that HFC cable systems could be used for two-way transmission of digital data. The data may be Internet data, digital audio data, or digital video data, in MPEG format, for example, from one or more external sources 100. Using two-way HFC cable systems for transmitting digital data is attractive for a number of reasons. Most notably, they provide up to a thousand times faster transmission of digital data than is presently possible over telephone lines. However, in order for a two-way cable system to provide digital communications, subscribers must be equipped with cable modems, such as cable modem 120. With respect to Internet data, the public telephone network has been used, for the most part, to access the Internet from remote locations. Through telephone lines, data is typically transmitted at speeds ranging from 2,400 to 56,600 bits per second (bps) using commercial (and widely used) data modems for personal computers. Using a two-way HFC system as shown in FIG. 1 with cable modems, data may be transferred at speeds of 10 million bps, or more. Table 1 is a comparison of transmission times for transmitting a 500 kilobyte image over the Internet.
Furthermore, subscribers can be fully connected twenty-four hours a day to services without interfering with cable television service or phone service. The cable modem, an improvement of a conventional PC data modem, provides this high speed connectivity and is, therefore, instrumental in transforming the cable system into a full service provider of video, voice and data telecommunications services.
As mentioned above, the cable industry has been upgrading its coaxial cable systems to HFC systems that utilize fiber optics to connect headends to fiber nodes and, in some instances, to also use them in the trunk lines of the coaxial distribution system. In way of background, optical fiber is constructed from thin strands of glass that carry signals longer distances and have a wider bandwidth than either coaxial cable or the twisted pair copper wire used by telephone companies. Fiber optic lines allow signals to be carried much greater distances without the use of amplifiers (item 114 of FIG. 1). Amplifiers degrade the signal quality and are susceptible to high maintenance costs. Thus, coaxial distribution systems that use fiber optics have much less need for amplifiers. In addition, amplifiers are typically not needed for fiber optic lines (item 106 of FIG. 1) connecting the headend to the fiber nodes.
In cable systems, digital data is carried over radio frequency (RF) carrier signals. Cable modems are devices that convert digital data to a modulated RF signal and converts the RF signal back to digital form. The conversion is done at two points: at the subscriber""s home by a cable modem and at a CMTS located at the headend. The CMTS converts the digital data to a modulated RF signal which is carried over the fiber and coaxial lines to the subscriber premises. The cable modem then demodulates the RF signal and feeds the digital data to a computer. On the return path, the operations are reversed. The digital data is fed to the cable modem which converts it to a modulated RF signal (it is helpful to keep in mind that the word xe2x80x9cmodemxe2x80x9d is derived from modulator/demodulator). Once the CMTS receives the RF signal, it demodulates it and transmits the digital data to an external source.
As mentioned above, cable modem technology is in a unique position to meet the demands of users seeking fast access to information services, the Internet and business applications, and can be used by those interested in cablecommuting (a group of workers working from home or remote sites whose numbers will grow as the cable modem infrastructure becomes increasingly prevalent). Not surprisingly, with the growing interest in receiving data over cable network systems, there has been increased focus on performance, reliability, and improved maintenance of such systems. In sum, cable companies are in the midst of a transition from their traditional core business of entertainment video programming to a position as full service providers of video, voice and data telecommunication services. Among the elements that have made this transition possible are technologies such as the cable modem.
A problem common to all upstream data transmission on cable systems, i.e. transmissions from the cable modem in the home back to the headend, is ingress noise which lowers the signal-to-noise ratio, also referred to as carrier-to-noise ratio of an upstream channel. Ingress noise can result from numerous internal and external sources. Sources of noise internal to the cable system may include cable television network equipment, subscriber terminals (televisions, VCRs, cable modems, etc.), intermodular signals resulting from corroded cable termini, and core connections. Significant sources of noise external to the cable system include home appliances, welding machines, automobile ignition systems, and radio broadcasts, e.g. citizen band and ham radio transmissions. All of these ingress noise sources enter the cable system over the coaxial cable line, which acts essentially as a long antenna. Ultimately, when cable systems are entirely optical fiber, ingress noise will be a far less significant problem. However, until that time, ingress noise is and will continue to be a problem with upstream transmissions.
The portion of bandwidth reserved for upstream signals is normally in the 5 to 42 MHz range. Some of this frequency band may be allocated for set-top boxes, pay-pre-view, and other services provided over the cable system. Thus, a cable modem may only be entitled to some fraction or xe2x80x9csub-bandxe2x80x9d such as between 200 kHz to 3.2 MHz. This sub-band is referred to as its xe2x80x9callotted band slicexe2x80x9d of the entire upstream frequency range (5 to 42 MHz). This portion of the spectrumxe2x80x94from 5 to 42 MHzxe2x80x94is particularly subject to ingress noise and other types of interference. Thus, cable systems offering two-way data services must be designed to operate given these conditions.
An emerging standard establishing the protocol for two-way communication of digital data on cable systems has been defined by a consortium of industry groups. The protocol, known as the Data Over Cable System Interface Specification (DOCSIS), specifies particular standards regarding the transmission of data over cable systems. With regard to the sub-band mentioned above, DOCSIS specifies that the bandwidth of a data carrier should generally be 200 KHz to 3.2 MHz. Further references to DOCSIS standards will be made in the specification.
As noted above, ingress noise, typically narrow band, e.g., below 100 kHz, is a general noise pattern found in cable systems. Upstream channel noise resulting from ingress noise adversely impacts upstream data transmission by reducing data throughput and interrupting service, thereby adversely affecting performance and efficient maintenance. One strategy to deal with cable modem ingress noise is to position the modem""s upstream data carrier in an ingress noise gap where ingress noise is determined to be low, such as between radio transmission bands. The goal is to position data carriers to avoid already allocated areas.
Ingress noise varies with time, but tends to accumulate over the system and is measured at the headend (ingress noise is uniform over the entire wire). In addition, while a particular frequency band may have been appropriate for upstream transmissions at the beginning of a transmission, it may later be unacceptably noisy for carrying a signal. Therefore, a cable system must attempt to identify noisy frequency bands and locate optimal or better bands for upstream transmission of data at a given time.
Block 104 of FIG. 1 represents a cable modem termination system connected to a fiber node 108 by pairs of optical fibers 106. The primary functions of the CMTS are (1) receiving signals from external sources 100 and converting the format of those signals, e.g., microwave signals to electrical signals suitable for transmission over the cable system; (2) providing appropriate media access control level packet headers for data received by the cable system, (3) modulating and demodulating the data to and from the cable system, and (4) converting the electrical signal in the CMTS to an optical signal for transmission over the optical lines to the fiber nodes.
FIG. 2 is a block diagram showing the basic components of a cable modem termination system (item 104 of FIG. 1). Data Network Interface 202 is an interface component between external data sources and the cable system. External data sources (item 100 of FIG. 1) transmit data to data network interface 202 via optical fiber, microwave link, satellite link, or through various other media. A Media Access Control Block (MAC Block) 204 receives data packets from Data Network Interface 202. Its primary purpose is to encapsulate the data packets with a MAC headers containing cable modem addresses according to the MCNS standard. A MAC address is necessary to distinguish data from the cable modems since all the modems share a common upstream path, and so that the system knows where to send data. Thus, data packets, regardless of format, must be mapped to a particular MAC address.
MAC block 204 transmits data via a one-way communication medium to a downstream modulator and transmitter 206. Downstream modulator and transmitter 206 takes the data in a packet structure and modulates it on the downstream carrier using, for example, QAM (Quadrature Amplitude Modulation)-64 modulation (other modulation techniques can be used, such as CDMA (Code Division Multiple Access), OFDM {Orthogonal Frequency Division Multiplexing}, and FSK (FREQ Shift Keying)). The return data is likewise modulated using, for example, QAM-16 or QSPK. These modulation techniques methods are well-known in the art.
QAM-16 transmits using a 4-bit symbol, and requires a relatively high signal-to-noise ratio to work such as about 25 dB. QPSK (Quadrature Phase Shift Keying), on the other hand, used for demodulating the upstream data, transmits using only a 2-bit symbol, but does not require as high a signal-to-noise ratio, and can work at around 15 dB. QPSK is also less expensive and requires less processing than QAM-16. The downstream data path is considered less hostile to the signal and sends data at a very high rate, typically using QAM-64 modulation. The data carried upstream, however, is more susceptible to noise and typically requires a more robust format, such as QAM-16 or QPSK.
It should be noted that optical fibers transmit data in one direction per wavelength and coaxial cables can transmit data in two directions. Thus, there generally is only one coaxial cable leaving the fiber node which is used to send and receive data, whereas there are two optical fiber lines or wave guide from the fiber node to the downstream and upstream modulators.
Downstream Modulator and Transmitter 206 converts the digital data packets to modulated downstream RF frames, such as MPEG or ATM frames, using quadrature amplitude modulation, e.g. QAM-64, forward error correcting (FEC) code, and packet interleaving. Data from other services, such as television, is added at a combiner 207. Converter 208 converts the modulated RF electrical signals to optical signals that can be received and transmitted by a Fiber Node 210. Each Fiber Node 210 can generally service about 500 subscribers depending on bandwidth. Converter 212 converts optical signals transmitted by Fiber Node 210 to electrical signals that can be processed by an Upstream Demodulator and Receiver 214. This component demodulates the upstream RF signal (in the 5-42 MHz range) using, for example, QAM-16 or QPSK. It then sends the digital data to MAC 204.
One method of locating better channels for the upstream data carrier (ingress noise mitigation) is manually monitoring a particular channel and gathering historical data with respect to the noise level on that channel over a certain period of time. The historical data is then used to compile statistics regarding noise level change on that channel. This requires that an engineer or other human operator monitor and gather the data manually to compile the statistics regarding noise on the channel. This method presumes that the noise level on a channel is xe2x80x9cregularxe2x80x9d and can be predicted if sufficient statistics are compiled. Even after this process is automated to some degree in terms of monitoring and data gathering, it assumes that the noise on a channel has a pattern that can be detected. This a faulty and unjustifiable assumption. In reality, the noise level on all channels are random and chaotic. Specifically, if multiple periodic, yet independent, functions drive a single system, its measured output has a chaotic output function. Furthermore, even if it were possible to extract some degree of regularity in noise patterns, this process is cumbersome, expensive, and requires a high degree of human intervention.
Another method, closely related, involves using a device for gathering data and processing statistics one frequency at a time. The device essentially accumulates data, hashes it, and derives statistics using probability functions. Once it finds a low-noise channel, the system changes its upstream carrier to that channel. However, the time between determining a potentially lower noise channel and using that channel is in the range of minutes to hours. Consequently, the data is likely outdated by the time the system uses it for locating an upstream data carrier.
Both of these methods assume that the cable system is relatively static. However, it is evident that noise in the cable system is far more chaotic in nature than static. There are hundreds of sources of noise that can interfere with the carrier-to-noise ratio of the upstream data. For example, time, weather, temperature, electrical conductivity in the atmosphere, people""s habits, and other disparate factors can contribute to ingress noise on an upstream data carrier.
Frequency hopping is another technique used in DOCSIS cable modems for avoiding noise, particularly narrow band ingress noise. With frequency hopping, the frequency (channel) of a data carrier, but not its symbol rate or modulation, is changed to avoid an interfering carrier. Frequency hopping may be conducted without performing xe2x80x9clookahead,xe2x80x9d that is, without analyzing the available frequencies before changing the frequency of the data carrier. This technique is termed xe2x80x9cblind frequency hop.xe2x80x9d Blind frequency hop does not require additional hardware. The carrier frequency is sequentially changed according to a pre-established channelization table or formula until the channel is restored to acceptable noise levels. In the latter case, this often consists of a starting frequency and step size. If the noise-free spectrum does not fit the preestablished table or formula, then there is a reduced chance of fitting data carriers in those regions. Consequently, the rate of transmission errors may not be optimally minimized or may remain at excessively high levels.
Blind frequency hop may be modified with xe2x80x9cmicroadjustmentsxe2x80x9d in the channelization table or formula to accomplish better data carrier fit (i.e., to locate sufficiently wide bands for data transmission between regions of narrow band ingress noise). This modification means that he step size is reduced. Microadjustment provides improved fit (more efficient use of the available frequencies) between noise ingress gaps, but at the expense of an increase in search steps, since many more blind microadjustment hops than full channel hops (such as are performed in blind channel hopping) are required to cover the same range of available frequencies. Thus, microadjustment slows the process of locating an appropriate upstream data transmission band.
Therefore, what is needed is a reliable and efficient method of locating channels for upstream data carriers that have low noise levels, thereby enabling deliberate and intelligent selection of an upstream data carrier in a network system utilizing cable modems.
To achieve the foregoing, the present invention provides methods and apparatuses for quickly and efficiently locating a frequency band for upstream transmission of data in a data communications network system utilizing cable modems. In a preferred embodiment, a cable modem termination system (CMTS) at the headend performs a lookahead in order to determine the noise power spectrum allocated to its upstream channels. Free bands (noise-free bands available for upstream data transmission) may then be determined based on the noise power spectrum and the upstream channel capacity requirements according to a process that is preferably implemented in computer code at the headend. The lookahead is preferably accomplished by using a spectrum analyzer, which is preferably incorporated into the CMTS at the headend, to collect a table of samples of the noise power versus frequency. This table of samples is input to a computational algorithm which determines free band sets for one or more preferred modulation formats rapidly and efficiently in software without the stepping of the upstream receiver frequency as in the blind frequency hop process. Once a band has been located, the CMTS assigns its frequency to an upstream channel and signals the cable modem of the frequency update.
In one aspect, the present invention provides a method of selecting an upstream data transmission frequency band in a network system utilizing cable modems. The method involves sampling noise power for a set of allocated upstream data transmission frequency bands when there is no data transmission. A set of noise bands is determined within the allocated frequency bands, the noise bands being bands in which noise exceeds a reliable transmission threshold for a modulation format. A set of free bands is determined within the allocated frequency bands, the free band set being the set difference of the allocated bands and the noise bands. A first-fit is then performed on the free bands for a channel to be transmitted in order to select an upstream data transmission frequency band having capacity for that channel.
In other aspects, the invention also provides a computer program product including a computer-usable medium having computer-readable program code embodied thereon relating to selecting an upstream data transmission frequency band in a network system utilizing cable modems, the computer-readable program code effecting steps for accomplishing a method in accordance with the present invention. Network systems utilizing cable modems capable of two-way transmission of data, and cable television system headends incorporating cable modem termination systems capable of receiving and implementing such computer-readable program code are also provided.
These and other features and advantages of the present invention may be realized by reference to the remaining portions of the specification and the accompanying figures which illustrate by way of example principles of the invention.